Satellite-based radio navigation systems have become widely adopted in many commercial and military applications. Exemplary systems in operation or development include the NAVigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS), the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), a European satellite navigation system called GALILEO, the wide area augmentation system (WAAS), and the local area augmentation system (LAAS). These systems permit a user with an appropriate direct sequence spread spectrum (DSSS) signal receiver to determine his or her position. Direct Sequence Spread Spectrum is a modulation technique where a pseudorandom noise sequence directly phase modulates a data-modulated carrier. The DSSS signal has a noise-like spectrum and appears to be noise to all but the intended receiver.
As an example, the GPS constellation has 24 operational satellites. These satellites are positioned in six different orbital planes such that at any time a minimum of six satellites are visible to any user on the surface of the Earth, except in the polar region. The satellites operate in near circular 20,200 kilometers (about 12,000 miles) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period.
Each satellite contains at least one atomic clock and transmits a navigation message that contains an accurate system time and its orbital position referenced to the atomic clock. The navigation message also contains clock behavior, status messages, and correction data such as ionospheric delay, time offset, etc. An almanac that gives the approximate data for each active satellite is also provided.
Each satellite transmits on two L-band frequencies: L1=1575.42 MHz and L2=1227.6 MHz. Three sets of pseudorandom noise (PRN or PN) ranging codes are in use: the coarse/acquisition (C/A) code, the precision (P) code, and the Y-code.
The C/A code set, also known as Gold code, has a 1.023 MHz chip rate. In spread spectrum technology, the term “chip” refers to a single bit of a pseudorandom sequence (PN-sequence) and the term “chip rate” refers to the rate at which bits of a PN-sequence are shifted. The Gold code therefore has a length of 1023 chips. The term “code” refers to the binary bit stream (the pseudorandom sequence) used to spread a signal over a wide range of frequencies for transmission. This spreading improves the accuracy of position estimation. Other advantages include interference rejection and low spectral power density, i.e., the power level at a given frequency.
A correlator at a receiver despreads this signal to the original data bandwidth by correlating it with a locally generated PN-sequence identical to and in synchronization with the PN-sequence used to spread the carrier at the radio transmitter, e.g., a GPS satellite vehicle (SV). Typically, this despreading occurs after the signal received at the antenna has been amplified and down-converted to a suitable low carrier frequency, also known as the intermediate frequency (IF). The hardware section associated with the amplification, down-conversion, and analog-to-digital conversion (ADC) is called the radio frequency (RF) stage. The other section, which processes the RF stage output and generates the position, velocity, and time information, is called the baseband (BB) stage.
It is necessary to acquire the satellite signal in order to determine the pseudorange or approximate distance to the navigation satellite from the receiver and to extract the navigation data. The Direct Sequence Spread Spectrum (DSSS) signal employed requires a perfect correlation of the received signal with a locally generated PN code in order to acquire the signal. Additionally, the local carrier frequency should be sufficiently close to the received signal frequency, in which the closeness of the local frequency to the received frequency depends upon the intended length of integration or correlation. In the exemplary case of GPS, the first or short time integration is done over a length of 1023 chips with an associated time duration of 1 ms. This requires a residual carrier frequency of less than 500 Hz. Any increase in this residual frequency will result in some of the samples within the correlation or integration length being phase reversed with a negative contribution to the integration value. This decreased integration value results in the receiver not being able to acquire the GNSS signal. This problem becomes more pronounced as the coherent integration length is increased. In an exemplary case where the integration length is increased to say 2 ms the residual frequency needs to be less than 250 Hz. Thus the range of residual frequency puts a constraint on the coherent integration length. In such cases non-coherent integration in which small length coherent integration powers are considered is used. However, this is an inefficient method to boost the signal to noise ratio and so coherent integration is usually preferred.
The received GNSS signal strength becomes weak due to receiver operation in indoors or when the signals are blocked as in the case of foliage or urban canyon. A long coherent integration, sometimes extending up to several hundred milliseconds, is needed to acquire the weak signal. In addition to this, several sequential correct signal confirmation stages may be required. The residue frequency error between the locally generated frequency and incoming IF signal from the RF module during the integration interval should be small and should not impact the acquisition sensitivity. As an example, a Fast Fourier Transform (FFT) with downsampling algorithm can be used for long coherent integration. In a case of, e.g., a 5120 ms length integration with a downsampling of 20 times, the resulting FFT points will be 256 with a corresponding frequency resolution of 0.2 Hz. That means that if the frequency change during the integration is more than 0.1 Hz, then the signal power will be dispersed to two or more frequency bins. This leads to the decrease of peak power and makes the acquisition or tracking sensitivity lower. Thus when the integration is long, the frequency change during the integration must be considered even though the receiver is static. The Doppler frequency change due to the satellite dynamics alone has an average value of 0.5 Hz/Sec with a maximum of 1 Hz/sec.
Thus under weak signal condition a long coherent integration may be required to acquire or track the signal. This long integration requires a large number of samples, which need to be stored in memory with a large storage capacity. Integrating such a large memory with the GNSS receiver on a single chip is problematic because the memory increases the cost and power consumption of the chip and takes up valuable real estate on the chip.
The GNSS receiver may be part of an infotainment system that also includes audio and video players as well as a host of other devices. Each of the devices in the infotainment system may require large amounts of memory at different times. For example, the audio player may require a large amount of memory when the GNSS receiver is not functioning or does not require a large amount of memory. On the other hand, the GNSS receiver may require a large amount of memory when other devices in the system are not in use.
Accordingly, there is a need for a multi-function platform comprising a large memory that is external to the GNSS receiver and can be shared among the receiver and other devices on the platform. This would optimize memory in the multi-function platform thereby reducing its the cost and size, as well as power consumption.